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IB DP Biology — Unit D: Continuity and Change | SL | D4.3 Climate Change
📋 What You Will Learn
- Identify the major greenhouse gases and explain how the enhanced greenhouse effect drives global warming
- Explain how rising CO₂ concentrations affect ocean chemistry and the consequences for marine organisms
- Analyse how climate change alters species distributions, phenology, and interspecific relationships within ecosystems
- Interpret data from ice cores, temperature records, and ecological studies as evidence for climate change
- Evaluate the biological consequences of habitat loss, coral bleaching, and phenological mismatch for biodiversity
- Apply understanding of feedback loops to assess how biological systems can amplify or dampen climate change
🔍 Guiding Question How does climate change threaten the continuity of life on Earth, and what biological evidence do we have that it is already reshaping ecosystems?
🌍 Why This Matters In 2016, the Great Barrier Reef experienced its most severe coral bleaching event on record — over 50% of shallow-water corals in the northern section died within months. The trigger was a marine heatwave of just 1–2°C above the seasonal maximum, sustained for several weeks. This is not a distant future scenario. It is already happening, and biology is the lens through which we understand both the mechanism and the consequence.
The Greenhouse Effect: A Problem of Energy Balance
The Sun drives life on Earth, but the planet must also lose energy — otherwise it would heat without limit. For billions of years, Earth maintained a stable energy balance: incoming solar radiation equalled outgoing infrared radiation. The atmosphere acted as a selective filter, transparent to incoming shortwave radiation but partially absorbing the longer-wave infrared re-emitted from the surface. This natural greenhouse effect keeps Earth approximately 33°C warmer than it would otherwise be — warm enough for liquid water, and therefore for life.
The problem is not the greenhouse effect itself. The problem is that human activity has dramatically increased the concentration of greenhouse gases, intensifying this absorption and disrupting the energy balance that life evolved within.
📖 Key Definition — Greenhouse Effect The process by which certain atmospheric gases absorb outgoing infrared radiation emitted by Earth's surface and re-emit it in all directions, including back toward the surface, thereby warming the lower atmosphere and surface beyond what solar radiation alone would produce.
📖 Key Definition — Enhanced Greenhouse Effect The intensification of the natural greenhouse effect caused by increased atmospheric concentrations of greenhouse gases resulting from human activities, leading to additional warming of Earth's surface and lower atmosphere.
The Major Greenhouse Gases
Not all atmospheric gases absorb infrared radiation. Nitrogen (N₂) and oxygen (O₂) — which together make up 99% of the atmosphere — are transparent to infrared. Greenhouse gases are those with molecular structures that allow them to absorb and re-emit infrared radiation. The key greenhouse gases in the context of climate change are carbon dioxide (CO₂), methane (CH₄), water vapour (H₂O), nitrous oxide (N₂O), and ozone (O₃).
- Carbon dioxide (CO₂): Released by combustion of fossil fuels, deforestation, and cement production. Atmospheric concentration has risen from approximately 280 ppm (pre-industrial) to over 420 ppm today — a 50% increase in under 300 years.
- Methane (CH₄): Released by livestock digestion (enteric fermentation), rice paddies, landfill decomposition, and natural gas leaks. Approximately 80 times more potent than CO₂ over a 20-year period, though shorter-lived in the atmosphere.
- Nitrous oxide (N₂O): Released from agricultural soils (nitrification and denitrification), synthetic fertilisers, and